Electrochemical solid electrolyte sensors, in particular for determining the oxygen content in exhaust gases of internal combustion engines, operate according to the so-called Nemst principle, according to which an electromotive force (EMF) is picked off, as the probe voltage, between a reference electrode having an excess of oxygen and a measurement electrode to which the measured gas is applied. The EMF occurs if an oxygen concentration λ<1 is present in the measured gas, stoichiometric conditions being present in the measured gas when λ=1. The probe voltage is conveyed to a control device as a measurement signal. Electrochemical solid electrolyte sensors require a temperature of at least 300° C. in order to operate. An electrical resistance heater, operated with a heating voltage that corresponds (when the sensor is used in a motor vehicle) to the vehicle's battery voltage, is integrated into the solid electrolyte sensor for that purpose. The reference electrode of the solid electrolyte sensor is connected as the positive electrode. The measurement electrode is connected to ground (negative pole). When solid electrolyte sensors are operated, it if found that coupling of the heat voltage into the probe voltage occurs. This falsifies the measurement signal. It has already been proposed to separate the sensing element and the heater from one another, or to provide between the heater and the adjacent electrode a shielding electrode to dissipate the coupled-in voltage (see German Patent Application No. 31 20 159).